1. Field of Invention
This invention relates to single-lens-reflex (SLR) cameras, specifically to an improved light reflection mechanism for an SLR camera using a liquid-crystal switch.
2. Description of Prior Art
The single-lens-reflex (SLR) camera is the most highly developed and popular camera. The basic idea is to use a hinged mirror to reflect the image formed by the lens onto a viewing screen until exposure. This enables a photographer to see the image exactly as it will be formed on film. When the photographer actuates the shutter, the mirror pivots up and away so that the light of the object photographed can image directly onto the back of the camera in synchronism with the opening of the shutter. After exposure the mirror pivots back. The main advantage of SLR structure is that in addition to enabling the photographer to see exactly what the film will see, the SLR will also enable the photographer to also see precise distance focused, the depth of field, and the field of view on the viewing screen.
A prior-art SLR mechanism is schematically shown in FIGS. 1 and 2. FIG. 1 shows the light path when a shutter 28 is closed. The incident light passes through a lens 20 and is reflected by a hinged mirror 22 to a prism 24. Thus the image formed by lens 20 can be viewed by a user 26 through prism 24 and related viewing optics.
FIG. 2 shows the light path when an exposure is taken. When the shutter release button of the camera is pressed, hinged mirror 22 pivots up, allowing light to reach the back of the camera. Then mechanical shutter 28 is opened for a desired, preset exposure time. Consequently, the image formed by lens 20 exposes film 30. After exposure, shutter 28 closes again, and hinged mirror 22 falls back into place (FIG. 1).
It is obvious that the SLR structure allows no light to be wasted in film exposure, since 100% of the incident light goes to film 30.
The SLR structure is especially useful for ophthalmic cameras, especially a fundus camera, which is designed to photograph the fundus (macular region or fovea) of the retina. This is because the intensity of the light reflected from the photographed eye region is very low. For example, only about two percent of incident light is reflected by retina. Also, the intensity of illuminating light must be relatively low in order not to damage the eye.
A schematic diagram of a prior-art fundus camera is shown in FIG. 3. A pilot lamp 32 illuminates a patient's eye 38 through a half-silvered mirror 34 and an objective lens 36. The light reflected from eye 38 passes again through objective lens 36. After passing through mirror 34, the collected light is focused by a focusing lens 40 onto viewer's eye 26 via hinged mirror 22 and prism 24. Before shutter 28 opens, mirror 22 flips up so that the light travels to shutter 28 so that when shutter 28 opens, lens 40 focusses the light onto film 30. Thus a user 26 can monitor the image of fundus before a picture is taken.
When the picture is taken, a strobe lamp 42 is pulsed on, causing an intense and short pulse of light (about 350 microseconds) illuminates patient's eye 38 and exposes film 30.
The mechanism of the SLR camera and its advantages have been known for long time. A large number of attempts for improving the mechanism of hinged mirror and mechanical shutter have been made. For example, U.S. Pat. No. 589,349 to Holst (1897) and U.S. Pat. No. 4,786,928 to Lawther et al. (1988) describe two of those attempts. Holst uses a mirror composed of two movable abutting parts to enable an SLR camera to take photographs with lenses having different focal lengths. Lawther uses a single downward-swinging mirror to reduce the required clearance behind the lens for the mirror movement. Since they still use moving mirrors, the mechanical motion of the mirror in their SLR cameras, as well as the mechanical motion of the mirror in other SLR cameras, generates uncomfortable sound and vibration that may degrade the quality of the picture taken.
Similarly, many improvements have been made on fundus cameras. However, those apply only to fundus cameras that employ the hinged mirror. For example, U.S. Pat. No. 4,756,613 to Okashita (1988) describes a fundus camera employing a pair of automated viewing targets. When a picture is taken, the motion of the hinged mirror generates vibration and sound that may blur the image of the fundus.
To summarize, the disadvantages of this prior-art SLR type fundus camera are as follows:
(a) The motion of hinged mirror 22 generates vibration. This requires that the strobe light exposure must wait until the vibration is negligible.
(b) The motion of hinged mirror 22 generates sound that may stimulate the motion of the patient's eye under exposure.
(c) The sound generated by hinged mirror 22 is uncomfortable for both the operator, a physician or a technician, and the patient.
(d) Hinged mirror 22 is fragile. Thus a fundus camera employing a hinged mirror does not last long.